Process and device for the catalytic cleaning of the exhaust gas from a combustion plant

ABSTRACT

Exhaust gas to be cleaned is introduced into a conversion and mixing duct and the exhaust gas flows through the duct along a predetermined longitudinal direction. A reducing agent, such as aqueous urea solution, is injected into the exhaust gas stream in the conversion and mixing duct. The exhaust gas stream is then deflected into a reaction duct which extends parallel to or coaxially around the conversion and mixing duct. The exhaust gas then flows in the opposite direction through the reaction duct. A reduction catalyst is disposed in the reaction duct, where the reducible components of the exhaust gas are reduced. The exhaust gas cleaned in this manner is then discharged from the reaction duct.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of copending international applicationPCT/DE96/01094, filed Jun. 21, 1996, which designated the United States.

BACKGROUND OF THE INVENTION Field of the Invention

The invention pertains to a process for catalytically purifying theexhaust gas of a combustion plant fueled with solid, liquid or gaseousfuel. A liquid or gaseous reducing agent, such as aqueous urea solutionor gaseous ammonia, is injected into the hot exhaust gas stream in theprocess. The injection is effected with an atomizing medium, such ascompressed air. The reducing agent is thereupon decomposed and acatalytic reduction (in particular by the SCR method) with the hazardousgaseous exhaust gas components is induced. The invention further relatesto a device for the catalytic cleaning of exhaust gas from a combustionplant.

The invention is used in the catalytic cleaning of exhaust gas, inparticular by the SCR process, of engines and fired plants, e.g. dieselengines, diesel-gas engines, gas engines in the lean mode, boilers firedwith oil, gas, wood and other fuels, and gas turbines which areprovided, for example, for driving generators, compressors, commercialvehicles, machines, ships and locomotives or, for example, forgenerating heat, steam and hot water.

U.S. Pat. No. 5,431,893 to Hug et al. (EP-B1 0 558 452), in particularFIG. 1 thereof, discloses a process and a device of above-mentioned typefor cleaning exhaust gases from combustion plants. The prior art devicecomprises a housing having three mutually parallel ducts or chambers. Afeed line for the hot exhaust gases to be cleaned opens laterally intothe first end of the first duct, which-is termed the pyrolysis duct. Atwo-component atomizing nozzle is disposed at the first end, roughlycentrally. To this is fed a reactant, such as aqueous urea solution, andan atomizing medium, such as compressed air at a gauge pressure of 1bar. The urea solution and the compressed air form an aerosol-like spraycone which is carried in the longitudinal direction of the pyrolysisduct. The urea is completely converted or broken down into finelydivided ammonia and carbon dioxide. The exhaust gas stream containingthe ammonia and the carbon dioxide passes, at the second end of thepyrolysis duct, into the second parallel duct, which is termed themixing duct. The exhaust gas stream runs there in the oppositedirection, more precisely through some conventional crossflow mixers.The exhaust gas stream, which is at that point homogeneously andintensively mixed with the broken-down reactant is subsequently passedinto the third parallel duct, which is termed the reaction duct. Thereit is first conducted through two reduction catalysts which are spacedapart a given distance from one another, preferably SCR catalysts(SCR=selective catalytic reduction). Then it is optionally conductedthrough an oxidation catalyst, likewise arranged at a distance thereto,and constructed with the same geometry. The reduction catalysts and theoptional oxidation catalyst are of a honeycomb structure, that isequipped with longitudinal ducts. The exhaust gas stream then freed fromall gaseous pollutants then passes into a heat exchanger or exits via anoutlet, e.g. a stack or exhaust.

That prior art device for cleaning exhaust gases can be constructed in ahighly compact system because of the parallel arrangement of the threeducts. However, for many applications, a still more compact constructionwith the same performance is desirable. This requirement is due to thefact that available space is often very limited, such as, for instance,in the case of motor vehicles.

German patent application DE-A1 42 03 807 discloses an exhaust gascleaning system with a centrally symmetrical structure. However, nodeflection is effected in that system. Due to the relatively excessivelength of that system, it cannot be used in all motor vehicle types, forexample. A more compact structure would be desirable.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method anddevice for catalytically purifying exhaust gas from a combustion plant,which overcomes the above-mentioned disadvantages of theheretofore-known devices and methods of this general type and whichallow for still more compact structures without sacrificing in terms ofsystem performance.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a process for cleaning exhaust gas from acombustion plant fueled with solid, liquid, or gaseous fuel. The processcomprises the following steps:

a) introducing exhaust gas into a combined conversion and mixing ductand forming an exhaust gas stream through the duct along a predeterminedlongitudinal direction;

b) injecting reducing agent into the exhaust gas stream flowing throughthe conversion and mixing duct, and mixing the reducing agent with theexhaust gas and simultaneously converting the reducing agent;

c) providing a reaction duct parallel to the conversion and mixing duct,and deflecting the exhaust gas stream from the conversion and mixingduct into the reaction duct and causing the exhaust gas to flow in adirection opposite the predetermined longitudinal direction;

d) contacting the exhaust gas with a reduction catalyst in the reactionduct and reducing reducible exhaust gas components of the exhaust gas;and

e) discharging a stream of cleaned exhaust gas from the reaction duct.

The invention is based on the finding confirmed by testing that thefirst and second duct can, under certain boundary conditions, befunctionally united, i.e., combined, to form a single duct, which is thecombined conversion and a mixing duct. The combined duct may extendparallel to the reaction duct or inside the reaction duct (preferablycentrally). A spatial separation into three ducts is therefore,surprisingly, not necessary for the proper function of the system.

In accordance with an added mode of the invention, the introducing stepcomprises forming a vortex flow in the exhaust gas stream uponintroducing the exhaust gas into the combined conversion and mixingduct.

In accordance with an additional feature of the invention, the injectingstep comprises injecting a reducing agent selected from the groupconsisting of liquid (e.g. aqueous urea solution) and gaseous (e.g.ammonia) reducing agents into the exhaust gas.

In accordance with another feature of the invention, the process furthercomprises a step of contacting the exhaust gas flowing in the reactionduct with an oxidation catalyst and/or contacting the exhaust gas with ahydrolysis catalyst.

With the above and other objects in view there is provided, inaccordance with the invention, a device for cleaning exhaust gasoriginating from the combustion of solid, liquid, or aqueous fuel,comprising:

a housing having a combined conversion and mixing duct defined thereinand being formed with an exhaust gas inlet for feeding exhaust gas intothe conversion and mixing duct, the conversion and mixing duct defininga longitudinal direction along which the exhaust gas flows through theconversion and mixing duct;

a feed apparatus for injecting a reducing agent into the conversion andmixing duct;

the housing having a reaction duct formed therein extendingsubstantially parallel to the conversion and mixing duct, and includinga reducing catalyst disposed in the reaction duct;

a deflector disposed between the conversion and mixing duct and thereaction duct, the deflector deflecting the exhaust gas from thelongitudinal direction in the conversion and mixing duct into anopposite direction in the reaction duct; and

the housing being formed with an outlet for outflowing cleaned exhaustgas from the reaction duct.

In accordance with again an added feature of the invention, the reactionduct surrounds the conversion and mixing duct externally.

In accordance with again another feature of the invention, the feedapparatus is a two-component atomizing nozzle for aqueous urea andcompressed air disposed centrally at the inlet of the conversion andmixing duct.

In accordance with again an additional feature of the invention, thereis provided a further deflector disposed between the reaction duct andthe outlet.

In accordance with again a further feature of the invention, there isprovided an oxidation catalyst disposed in the reaction duct.Furthermore, a hydrolysis catalyst may be disposed in the housingupstream of the reduction catalyst, relative to a flow of the exhaustgas. The hydrolysis catalyst is disposed in the conversion and mixingduct, in the deflector, or in the reaction duct.

In accordance with yet an added feature of the invention, the conversionand mixing duct is disposed centrally within the reaction duct.

In accordance with yet an additional feature of the invention, thereduction catalyst (e.g. an SCR catalyst) is a catalytic converterformed with individual modules, and the conversion and mixing duct has across-section selected from the group consisting of rectangular, round,and rectangular with rounded corners. The individual modules have aparallelepiped and/or circular-segment cross section.

In accordance with yet another feature of the invention, the housing isformed with a prechamber disposed between the exhaust gas inlet in thehousing and the conversion and mixing duct.

In accordance with yet a further feature of the invention, theconversion and mixing duct has an intake formed with at least two inletorifices for the exhaust gas.

In accordance with yet a supplemental feature of the invention, thedeflector(s) is provided with a catalytically active surface.

In accordance with yet another feature of the invention, there isprovided a silencer in the reaction duct.

With the principle of this invention, the reaction duct can be arrangedparallel and adjacent to the conversion and mixing duct (“paralleltype”). However, in a particularly preferred embodiment, the conversionand mixing duct is arranged in the middle area, specifically centrallyin the reaction duct (“central type”). Accordingly, a catalytic reactorconfiguration results in this invention with individual catalyst modulesthat make up the system. In the middle area—preferably centrally—theinner duct (the conversion and mixing duct) is formed through which theexhaust gas flows. The preferred catalytic reactor has catalyst modulesarranged in p rows and in q columns and the middle area—preferablycentrally—leaves free (m×n) catalyst modules to form the conversion andmixing duct. The variables m, n, p, and q are integers and m<p and n<q.The variables m and p pertain to the same direction (e.g. in theCartesian system) and the variables n and q pertain to the samedirection (orthogonal to m, p). According to another advantageousconstruction, a catalyst of circular ring shape is provided which iscomposed of a number of circular-segment catalyst modules whose innerregion is provided to form a conversion and mixing duct of round crosssection and which can be inserted into a housing of round externalshape. In one specific configuration, the catalytic reactor body isrounded off externally, where parallelepiped and circular-segmentmodules are used.

By means of the device according to the invention, a particularlycompact structure can be achieved, since only two mutually parallelducts or only two ducts arranged one inside the other are required. Thevolume of the structure can therefore be kept small in relation to thecatalyst volume. Owing to the highly simple, virtually symmetrical flowdistribution in preferred embodiments, the construction principle can beretained for different sizes. There is therefore the potential forretaining the construction principle (“scale-up”), which is highlydesirable for obvious cost reasons.

There is also provided, in accordance with the invention, a combinationof the device for cleaning exhaust gas and a catalytic converter. Thedevice has a combined conversion and mixing duct defined therein inwhich exhaust gas is mixed with a reducing agent injected into theconversion and mixing duct, a reaction duct extending substantiallyparallel to the conversion and mixing duct, and a deflector disposedbetween the conversion and mixing duct and the reaction duct, thedeflector deflecting the exhaust gas from a flow direction in theconversion and mixing duct into an opposite flow direction in thereaction duct. The catalytic converter comprises a plurality of catalystmodules defining an inner duct in a middle area thereof, the exhaust gasflowing through the inner duct.

In accordance with another feature of the invention, the inner duct isdisposed centrally within the plurality of catalyst modules.

In accordance with an additional feature of the invention, the modulesare rectangular catalyst modules disposed in a checkered pattern along prows and q columns, and wherein, in the middle area, m×n modules areomitted to form the inner duct, and wherein m, n, p, and q are integers,m<p, and n<q.

In accordance with again another feature of the invention, the modulesare circular-segment catalyst modules defining a circular ring shape ofthe catalytic converter, wherein the inner duct has a round crosssection, and wherein the catalytic converter is insertible into ahousing having a round outer contour.

In accordance with a concomitant feature of the invention, the modulesare a plurality of rectangular catalyst modules and a plurality ofcircular-segment catalyst modules defining a substantially rectangularcatalytic converter with rounded corners.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a process and device for the catalytic cleaning of the exhaust gasfrom a combustion plant, it is nevertheless not intended to be limitedto the details shown, since various modifications and structural changesmay be made therein without departing from the spirit of the inventionand within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a “parallel type” catalytic cleaning devicewith two parallel ducts, taken along the line I—I in FIG. 2, and viewedin the direction of the arrows;

FIG. 2 is a longitudinal section of the cleaning device, taken along theline II—II of FIG. 1;

FIG. 3 is a longitudinal section of a first central type catalyticcleaning device with two ducts arranged centrally relative to oneanother;

FIG. 4 is an elevational view of the intake side of the cleaning deviceof FIG. 3;

FIG. 5 is a longitudinal section of a second central type catalyticcleaning device with two ducts arranged centrally relative to oneanother;

FIG. 6 is a section of the cleaning device of FIG. 5, taken along theline V—V;

FIG. 7 is a diagrammatic cross section of an arrangement of 72parallelepipedal catalyst modules in the reaction duct, leaving clear arectangular central conversion and mixing duct;

FIG. 8 is a similar view of an arrangement of four circular-segmentcatalyst modules in the reaction duct, with a round central conversionand mixing duct; and

FIG. 9 is a similar view of an arrangement of four parallelepipedal andfour circular-segment catalyst modules in the reaction duct, with arectangular central conversion and mixing duct.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawing in detail and first,particularly, to FIGS. 1 and 2 thereof, a device for catalyticallypurifying (cleaning) the exhaust gas a of a combustion plant has aparallelepipedal housing 10. Two feed lines 12 a (only one of which isshown) for the exhaust gas a, which may have a temperature of 200 to500° C., lead into this housing 10. For simplicity, only the exhaust gasfeed line 12 a is shown in FIG. 2. The second exhaust gas feed linecannot be seen in FIG. 2 because it is blocked by feed line 12 a becausethe figure is symmetrically arranged about line II of FIG. 1 relative togas feed line 12 a.

Intake orifices 14 a and 14 b associated with the two exhaust gas feedlines 12 a and 12 b are shown in dashed lines for clarity in FIG. 1. Theintake is generally identified at 14. The two intake orifices 14 a, 14 blead into a wide, elongated injection chamber that will be referred to aconversion and mixing duct the conversion and mixing duct 16 has aparallelepipedal cross section. Its longitudinal axis is designated 18.The two exhaust gas feed lines 12 a, 12 b are orientated obliquely tothe longitudinal axis 18. In the exemplary embodiment, they are alsoorientated at an angle to the plane of the paper and in oppositedirections. This generates an intake swirl in the inflowing exhaust gasa. A partition 20 extending in the longitudinal axis 18 also serves todefine the conversion and mixing duct 16.

A two-component atomizer nozzle 22 (feed apparatus) is centrallysituated at the inlet end of the conversion and mixing duct 16. Thenozzle 22 is disposed symmetrically in between the two intake orifices14a and 14 b. It is operated with a reducing agent r (e.g. aqueous ureasolution) and with an atomizing medium p (e.g. compressed air). Thespray cone generated on operation is designated 24. The cone angleshould be between 10° and 40°, preferably greater than 20°. Thetwo-component atomizer nozzle 22 can be adapted in this case byselecting spray cone, mean droplet diameter, range of throw andevaporation section, in such a manner that a substantial conversion ofthe urea into ammonia, evaporation of the same and mixing with theinflowing exhaust gas a takes place in the conversion and mixing duct16.

An optional hydrolysis catalytic converter 28 is disposed in theconversion and mixing duct 16. It will be appreciated that the catalyticconverter 28 is provided when, as a result of the required minimumresidence time of the urea and owing to other process parameters, thelength of the combined conversion and mixing duct 16, to ensure completereaction, would be too long. The hydrolysis catalyst 28 likewise effectsa certain mixing of the inflowing substances, so that after itsinstallation, the conversion and mixing duct 16 with a reduced length issufficient. It is important that the urea is virtually completelyconverted into ammonia at the end of the conversion and mixing duct 16.

A first deflector 30 is disposed at the outflow end of the conversionand mixing duct 16. Non-illustrated deflection devices or deflectionbaffles may be provided in the first deflector 30. The first deflector30 deflects the flowing substances by 180° and into the opposite flowdirection 32.

The deflected substances enter a reaction duct 34. In this reaction duct34, at least one reduction catalyst 36, preferably an SCR catalyst, isaccommodated. Two reduction catalysts 36, 38 (catalyst blocks), arrangedat a distance to one another, are installed there in the present case,through the longitudinal ducts of which the substances applied flow inthe opposite direction 32. The reduction catalysts 36, 38 serve in thiscase in a known manner for the conversion of undesirable constituents orcomponents of the exhaust gas a, but in particular for the destructionof the harmful nitrogen oxides NO_(x).

It is important for the compact structure of the housing 10 that theflow along the longitudinal axis 18 of the exhaust gas a in theconversion and mixing duct 16 runs in the opposite direction to thedirection of flow 32 in the reaction duct 34 and thus in the reductioncatalysts 36, 38.

As shown in FIG. 1, the reduction catalyst 38 in this exemplaryembodiment is made up of six identical modules placed one on top of theother, just as is the reduction catalyst 36.

The second reduction catalyst 38 can be followed with or without aspacing distance by an oxidation catalyst 39 of identical crosssectional dimensions. The oxidation catalyst 39 catalyzes the conversionof CO, C_(n)H_(n) and other oxidizable exhaust gas constituents.

A second deflector 40 follows at the outlet of the reaction duct 34.This second deflection effects a deflection of 90° in the directiontoward an outlet orifice 42, at which there is situated an outlet 44 forthe cleaned exhaust gas a′.

It must be emphasized here once again: the conversion and mixing duct 16serves here as evaporation, mixing and conversion section. No staticmixers are present or required in this conversion and mixing duct 16 tomix the evaporating ammonia NH₃ intensively with the exhaust gas a. Thisis considered to be a significant advantage.

Nevertheless, reaction and uniform mixing of injected reducing agent rand exhaust gas a are ensured.

It is further important that the two exhaust gas feed lines 12 a, 12 bare arranged symmetrically about the longitudinal axis 18 of theconversion and mixing duct 16. This disposition results in good mixingof the components a, p and r.

FIGS. 3 and 4 show a preferred embodiment of a catalytic cleaning devicewith inflow of the exhaust gas a into a central first duct and backflowthrough a second duct which embraces the first duct externally andcoaxially therewith. In this embodiment, an intake or exhaust gas feedline 12 leads obliquely into an annular prechamber 50. The prechamber 50could also be constructed to have a circular disk shape. As FIG. 4shows, in particular, the exhaust gas feed line 12 is arranged at anangle, more precisely in such a manner that the exhaust gas a is putinto a rotating (vortex) motion with respect to the longitudinaldirection 18 inside the prechamber 50. The annular prechamber 50,centrally to the longitudinal axis 18, has an annular outlet orifice 52which is formed using a preassembled wall 54. The outlet orifice 52 issimultaneously the inlet for the conversion and mixing duct 16. A feeddevice 22, that is a two-component atomizer nozzle for reducing agent rand atomizing medium p, is attached to the preassembled wall 54. Thespray cone of the atomizer nozzle 22 is designated 24. The structure ismade in such a way that a certain free space 56 is present externallyfor assembly.

The exhaust gas a flows via the outlet orifice 52 into the conversionand mixing duct 16 and mixes there with the atomized reducing agent r.

An optional hydrolysis catalyst 28 may be disposed in the conversion andmixing duct 16.

At the end of the conversion and mixing duct 16, there is also situatedhere a deflector or deflection chamber 58. It reaches over an annularreaction duct 34 which externally surrounds the conversion and mixingduct 16 and is virtually completely filled with a reduction catalyst 36.Its longitudinal ducts run in parallel to the longitudinal axis 18. Toguide the gas mixture, deflection plates or deflection guides 60 can bepresent in the deflection chamber 58. The conversion and mixing duct 16can be formed by a tubular sheet wall 61. With certain constructionalpreconditions relating to the mounting of the reaction catalyst 36 takeninto account, the sheet wall 61 can be omitted. The gas in theconversion and mixing duct 16 is then already exposed to the catalyticmaterial on the surface of the catalyst 36.

The back wall 62 of the deflection chamber 58 and the conversion andmixing duct 16, in the present case, therefore, the sheet wall 61, canbe internally coated with a catalytically active surface. Likewise, thedeflection guides 60 can be coated with a catalytically active layer ofthis type. This can be a hydrolysis-catalyzing or SCR-catalyzing layer.It has been found that a coating of this type at these points canprevent precipitation of solids and thus blockage.

After flowing through the conversion and mixing duct 16, which may beannular with a rectangular or round cross section, the gas mixturepasses into an annular outflow chamber 64 of the same geometry. Theoutflow chamber 64 is, in the present case, provided with two outflowducts or outlets 66, 68; in principle, one outlet would also besufficient. From here, the catalytically cleaned exhaust gas a′ can beconducted, if appropriate via further apparatuses (such as heatexchangers, downstream silencers) in the exhaust gas duct, to an exhaustor stack.

The embodiment according to FIGS. 5 and 6 is substantially similar tothat of FIGS. 3 and 4. However, the outlet chamber or outflow chamber 64is not directly provided with one or more outlets in this case. Rather,further deflection takes place at the outlet chamber 64. In other words,the outlet chamber 64 is designed in such a way that the gas stream isdeflected in parallel to the longitudinal axis 18 into a silencingchamber 70. The silencing chamber 70 is an annular space which liesoutside the conversion and mixing duct 16 containing the SCR catalyst36. This silencing chamber 70 is, in the present case, provided with twoexhaust gas exits or outlets 72, 74, from which the cleaned exhaust gasa′ is conducted away. In principle, one outlet would be sufficient hereas well. The outer wall of the housing 10 can be additionally coatedwith a non-illustrated sound-absorbing material in order to reinforcethe silencing function of the silencing chamber 70.

In the present case also, a catalytically active coating, preferably anSCR coating, can be provided in the deflection chamber 58 and in theconversion and mixing duct 16.

The two-component atomizing nozzle 22 for the reducing agent r and theatomizing medium p can also in the present case be a commercial nozzle,the spray cone 24 of which forms a preset spray angle. It should bethermally insulated with respect to the housing 10. Superb mixing isinduced in this embodiment as well, owing to the swirl which the exhaustgas experiences about the longitudinal axis 18.

In this embodiment, also, the SCR catalyst 36 is formed from individualmodules 362. This becomes particularly apparent from FIG. 6. Accordingto this, a total of (3×3)−1=8 such modules having standard dimensionsare provided. The modules each have a rectangular, specifically asquare, cross section. Instead of this, a rotationally symmetricalconfiguration may also be chosen. Owing to the modular constructionpredetermined here, the conversion and mixing duct 16 also has arectangular, here specifically a square, cross section in the rightpart. On the left part, in accordance with FIG. 6, the intake area(chamber 50) can be round or rounded. The flow therefore, seen in thedirection of flow, changes from a round to a square cross section. Thedevice shown for cleaning the exhaust gas a is designed in particularfor use in combination with the diesel engine of a truck.

The following should be further noted with respect to the reductioncatalyst 36: it can comprise individual modules (as shown) each stackedone on top of the other, or a plurality of modules arranged one afterthe other. The individual modules, in the case of the sequentialarrangement, can have an intermediate space, which has already beenshown in FIG. 2. The intermediate spaces can separate differentcatalysts from one another. For example, firstly an SCR catalyst moduleand then an oxidation catalyst module can be respectively used.

FIG. 7 shows a somewhat different type of structure of the catalyst 36.In this case, modules are arranged in p=9 rows and q=9 columns. In themiddle (a central structure is present here) a space for the conversionand mixing duct 16 is left clear. For this, (m×n)=3×3 modules areomitted, so that in total (p×q)−(m×n)=9×9−9=72 modules are distributedover the cross section of the reaction duct 34.

Obviously, arrangements having 3×3−1=8, 4×4−2×2=12, 5×5−3×3=16,6×6−2×2=32, 7×7−3×3=40, 8×8−4×4=48, 10×10−4×4=84 etc. catalyst modulescan also be used as alternatives. In this case, it has been selected ineach case that p=q and m=n. Furthermore, not only are square structurespossible, but, in principle, other rectangular structures are alsopossible, e.g. having (7×5)−(3×3)=26 modules. p≠q and/or m≠n cantherefore also be selected.

It becomes clear from this that a rectangular structure can generallyhave the configuration (p×q)−(m×n) (where p>m, q>n and p, q, m, n areeach integers). The duct 36, for which the (m×n) modules have beenomitted, is preferably a central duct (“central structure”). However, itneed not lie centrally.

However, it must also be emphasized that an overall round or elserounded structure is also possible for the catalyst 36 and thus for thehousing 10. In other words: this structure can have a generally round orrectangular conversion and mixing duct 16, one or more annular catalysts36 and a round outer contour. The annular catalyst 36 is thenexpediently composed of a number of segmental catalyst modules 362. Forexample, it can be composed of four quarter-circle segments.

In FIG. 8, a central round conversion and mixing duct 16 is provided. Inthe surrounding duct 34, which is formed between the tubular sheet wall61 and the shell or housing 10, there are four identical quarter-circlesegmental catalyst modules 36 r. In departure therefrom, more modules 36r can also be used, for example eight modules 36 r, which each extendover 45°. In the spaces between the individual modules 36 r, an adhesiveor sealing material, e.g. a ceramic-fiber material, is preferablyaccommodated. Swelling mats can also be used. These would then surroundthe modules 36 r in the radial direction. The round structure shown hasgood strength properties.

FIG. 9, in contrast, is a rounded structure. Here, the catalyst 36comprises four identical rectangular catalyst modules 36 z and fouridentical quarter-circle segmental catalyst modules 36 r which arerespectively arranged in alternation. Larger designs can include moremodules 36 z and 36 r. For example, p>3 rows (e.g. p=12) and q>3 columns(e.g. q=12) can also be provided here. In each of the corners there isthen one quarter-circle segmental module 36 r or a plurality ofsegmental modules 36 r. The gaps between the modules 36 r can likewisebe filled with an adhesive or sealing material. Swellable mats can alsobe used. These embodiments having rounded corners are also particularlystable to mechanical stress.

The reactors (catalyst configurations) of FIGS. 7 to 9 can preferably beused in particular for relatively large plants.

The central-symmetrical construction of the embodiments shown in FIGS. 3to 9 give the following advantages:

good mixing of the reducing agent with the exhaust gas;

good uniform distribution of the exhaust gas admixed with the reducingagent onto the catalyst(s);

potential for retaining the construction principle with different sizes(scale-up);

potential for modular structure in staged sizes of the same constructionprinciple, use of identical types of components (in particular ofcatalyst modules, of conversion and mixing ducts);

potential for installation of a hydrolysis catalyst in the (central)intake area (SCR process with urea);

potential for introducing deflection guides 60;

potential for fabricating the deflection guides 60 or the rear wall 62with catalytically active surfaces, e.g. for hydrolysis or for othercatalytic cleaning of exhaust gas;

potential for arranging combined catalysts in the conversion and mixing16, e.g. of SCR and oxidation catalysts;

good silencing by deflection to rear wall 62 and/or deflection guide 60and multiple cross section change/reflective surfaces. Potential forintroducing additional silencing internals or for shaping the flow ductsin such a way that the effect of a reflection silencer is achieved byinterference formation;

use of a two-component atomizing nozzle 22 with appropriate spray angle(approximately 10° to 40°) for injecting a reducing auxiliary, e.g.ammonia or urea, or another reducing agent releasing NH₃;

minimizing heat losses by substantial reduction of outer surface area asa result of compact structure;

equalization of the temperature at the critical point for thehydrolysis, that is downstream of the injection;

compact construction by arranging the conversion and the mixing duct inthe center of the catalyst space; and

good mixing and good reaction conversion rate, since each deflection ofthe exhaust gas a causes a mixing effect.

We claim:
 1. A device for cleaning exhaust gas, a housing having asingle duct being a combined conversion and mixing duct defined thereinand being formed with an exhaust gas inlet for feeding exhaust gas intosaid single duct, said single duct defining a longitudinal axis alongwhich the exhaust gas flows through said single duct, said exhaust gasinlet being obliquely angled to the longitudinal axis; a feed apparatusfor injecting a reducing agent into said single duct; said housinghaving a reaction duct formed therein extending substantially parallelto said single duct, and including a reducing catalyst disposed in saidreaction duct; a deflector disposed between said single duct and saidreaction duct, said deflector deflecting the exhaust gas from thelongitudinal direction in said single duct into an opposite direction insaid reaction duct; and said housing being formed with an outlet foroutflowing cleaned exhaust gas from said reaction duct.
 2. The deviceaccording to claim 1, wherein said reaction duct surrounds said singleduct externally.
 3. The device according to claim 1, wherein said feedapparatus is a two-component atomizing nozzle for aqueous urea andcompressed air disposed centrally at said inlet of said single duct. 4.The device according to claim 1, which further comprises a furtherdeflector disposed between said reaction duct and said outlet.
 5. Thedevice according to claim 1, which further comprises an oxidationcatalyst disposed in said reaction duct.
 6. The device according toclaim 1, which further comprises a hydrolysis catalyst disposed in saidhousing upstream of said reduction catalyst, relative to a flow of theexhaust gas.
 7. The device according to claim 6, wherein said hydrolysiscatalyst is disposed in one of said single duct, said deflector, andsaid reaction duct.
 8. The device according to claim 1, wherein saidsingle duct is disposed centrally within said reaction duct.
 9. Thedevice according to claim 1, wherein said reducing catalyst is acatalytic converter formed with individual modules, and said single ducthas a cross-section selected from the group consisting of rectangular,round, and rectangular with rounded corners.
 10. The device according toclaim 9, wherein said individual modules have a cross section selectedfrom the group consisting of parallelepiped and circular-segment. 11.The device according to claim 1, wherein said reduction catalyst is anSCR catalyst.
 12. The device according to claim 1, wherein said housingis formed with a prechamber disposed between said exhaust gas inlet insaid housing and said single duct.
 13. The device according to claim 1,wherein said single duct has an intake formed with at least two intakeorifices for the exhaust gas.
 14. The device according to claim 1,wherein said deflector is provided with a catalytically active surface.15. The device according to claim 4, wherein said further deflector isprovided with a catalytically active surface.
 16. The device accordingto claim 1, which further comprises a silencer in said reaction duct.17. In combination with a device for cleaning exhaust gas, the devicehaving a single duct being a combined conversion and mixing duct with alongitudinal axis defined therein and an exhaust gas inlet beingobliquely angled to the longitudinal axis and a feed apparatus forinjecting a reducing agent in the single duct, a reaction duct extendingsubstantially parallel to the single duct, and a deflector disposedbetween the conversion and mixing duct and the reaction duct, thedeflector deflecting the exhaust gas from a flow direction in the singleduct into an opposite flow direction in the reaction duct, a catalyticconverter, comprising: a plurality of catalyst modules being locafed inthe reaction duct and surroanding the conversion and mixing duct. 18.The combination according to claim 17, wherein said inner duct isdisposed centrally within said plurality of catalyst modules.
 19. Thecombination according to claim 17, wherein said modules are rectangularcatalyst modules disposed in a checkered pattern along p rows and qcolumns, and wherein, in the middle area, m×n modules are omitted toform said inner duct, and wherein m, n, p, and q are integers, m<p, andn<q.
 20. The combination according to claim 17, wherein said modules arecircular-segment catalyst modules defining a circular ring shape of saidcatalytic converter, wherein said inner duct has a round cross section,and wherein the catalytic converter is insertible into a housing havinga round outer contour.
 21. The combination according to claim 17,wherein said modules are a plurality of rectangular catalyst modules anda plurality of circular-segment catalyst modules defining asubstantially rectangular catalytic converter with rounded corners.